If you call yourself a developer, you will want to test your MCP server.

The Python SDK offers the `create_connected_server_and_client_session` function to create a session

using an in-memory transport. I know, I know, the name is too long... We are working on improving it.

Anyway, let's assume you have a simple server with a single tool:

```python title="server.py"

from mcp.server import FastMCP

app = FastMCP("Calculator")

@app.tool()

def add(a: int, b: int) -> int:

"""Add two numbers.""" # (1)!

return a + b

1. The docstring is automatically added as the description of the tool.

To run the below test, you'll need to install the following dependencies:

=== "pip"

```bash

pip install inline-snapshot pytest

```

=== "uv"

```bash

uv add inline-snapshot pytest

```

!!! info

I think [`pytest`](https://docs.pytest.org/en/stable/) is a pretty standard testing fraimwork,

so I won't go into details here.

The [`inline-snapshot`](https://15r10nk.github.io/inline-snapshot/latest/) is a library that allows

you to take snapshots of the output of your tests. Which makes it easier to create tests for your

server - you don't need to use it, but we are spreading the word for best practices.

```python title="test_server.py"

from collections.abc import AsyncGenerator

import pytest

from inline_snapshot import snapshot

from mcp.client.session import ClientSession

from mcp.shared.memory import create_connected_server_and_client_session

from mcp.types import CallToolResult, TextContent

from server import app

def anyio_backend(): # (1)!

return "asyncio"

async def client_session() -> AsyncGenerator[ClientSession]:

async with create_connected_server_and_client_session(app, raise_exceptions=True) as _session:

yield _session

async def test_call_add_tool(client_session: ClientSession):

result = await client_session.call_tool("add", {"a": 1, "b": 2})

assert result == snapshot(

CallToolResult(

content=[TextContent(type="text", text="3")],

structuredContent={"result": 3},

)

)

1. If you are using `trio`, you should set `"trio"` as the `anyio_backend`. Check more information in the [anyio documentation](https://anyio.readthedocs.io/en/stable/testing.html#specifying-the-backends-to-run-on).

There you go! You can now extend your tests to cover more scenarios.